Thermal electromotive force (EMF) is a voltage that is generated when two dissimilar metals are joined together. When there are two of these junctions that are of opposite polarity and the temperature of the junctions are equal, there is no net voltage. When one of the junctions is at a different temperature than the other, a net voltage difference can be detected. A resistor may have a metal resistive element connected between copper terminals, thereby providing two junctions and making the resistor susceptible to adverse effects of thermal EMF.
Resistors of this construction are often used to sense current by measuring the voltage drop across the resistor. In cases where the current is low, the signal voltage generated across the resistor is also very small and any voltage caused by thermal EMF can cause a significant measurement error.
One prior art approach to addressing this problem has been to change the metal alloy used for the resistive element to one with a lower thermal EMF. In some cases this presents other challenges such as increased cost, an increase in bulk resistivity that creates a resistor geometry that is costly to manufacture, or sacrifices other electrical characteristics such as TCR (temperature coefficient of resistance).
Another prior art approach has been to add an ASIC (application specific integrated circuit) that is programmed to compensate for the offset voltage created by the thermally induced EMF. Such an approach adds material cost, complexity to the assembly, and manufacturing cost in terms of assembly steps and equipment.
What is needed is to provide a resistor that mitigates the effects of thermal EMF while not imposing constraints on the type of metal resistance alloy used.